Fault phenomena which occur in modern internal combustion engines and their regulating systems, for example aging of actuators and sensors, fuel leaks, sticking valves, carbonization of nozzle hole and other deposits, leakage currents etc., generally bring about undesired vehicle behavior, such as loss of power, increased emissions or else also an activated fault memory lamp. These fault phenomena can be due to both the air quantity regulation, the exhaust gas recirculation, the hydraulic system and also the electrical system. It is possible for both the sensors and the actuators to be affected. Owing to the high level of complexity of modern injection systems, a direct 1:1 assignment of causes to fault symptoms is often not possible. A fault phenomenon can have a plurality of causes. One cause can lead to a plurality of faults. In particular in the dynamic operating mode, on-board diagnostic strategies only make it possible to a limited extent to identify the cause of the fault in the injection system in a more detailed way, let alone to determine said cause precisely without at the same time having an adverse effect on the system behavior within the scope of the diagnosis. Intrusive tests during the operation of the vehicle are also not desired by the approval authority and/or by the manufacturers since they possibly cause the exhaust gas behavior to be worsened or can be perceived by the driver. In addition, localizing the cause of the fault by a limited number of available on-board sensor information items is restricted.
Due to a lack of precise knowledge of the cause of the fault, this leads in a workshop to high expenditure, often undesired, and as a consequence thereof possibly to components which are still functionally capable per se being unnecessarily replaced or too many components being replaced (trial & error approach). For example, this can result in a functionally capable ECU being replaced or an entire injector set being replaced even though the undesired system behavior was caused, for example, by a single defective injector or a soiled plug in the cable harness.
In addition, an initially moderate fault can, if not discovered, develop over time into a serious fault. This usually results in total failure of, for example, the injection system and therefore in the vehicle becoming immobilized.
DE 10 2006 036 567 B4 discloses a method for determining a functional state of a piezo-injector of an internal combustion engine. In this context, the input variables of a control loop for injecting fuel are the voltage and the charge. Taking a new capacitance and the last stored capacitance values as a basis, the further capacitance profile for the measured piezo-injector is calculated using a mathematical approximation method. An imminent failure of a piezo-injector is detected from the fact that a measured capacitance value is outside a first upper and lower tolerance range around the calculated capacitance profile. The piezo-injector is switched off immediately if the measured capacitance value is outside a second upper and lower threshold range around the calculated capacitance profile, wherein the threshold range includes the tolerance range.
As described above, in a workshop components are often replaced on the basis of suspicion. Alternatively, in a workshop additional sensor systems can be provided for diagnostic purposes and/or manual tests can be performed. Manual interventions into the injection system can, however, lead to a situation in which impurities penetrate the system and components are damaged.
DE 10 2005 040 551 B4 discloses a method for determining a proportion of biodiesel in a fuel for operating a diesel internal combustion engine, in which method the excess air ratio lambda in the exhaust gas of the diesel internal combustion engine is measured, an expected value of the excess air ratio lambda is determined by calculation on the basis of a measured air mass flow rate MAF and a calculated fuel quantity MF, wherein the mathematical relationship lambda=MAF/(14.5×MF) is used, and the proportion of biodiesel is determined from a difference between the measured excess air ratio lambda and the expected value of the excess air ratio lambda which is determined by calculation.
WO 2010/089236 A1 discloses a fault analysis method for an internal combustion engine having a plurality of cylinders. In this known method, an angular speed of the internal combustion engine is determined. Furthermore, a parameter of the combustion process of one of the plurality of cylinders is adapted in order to approximate the times at which the internal combustion engine respectively passes through an angle interval. In order to provide a fault analysis method which makes it possible to detect a defective cylinder, it is determined on the basis of the value of the parameter that the one cylinder of the plurality of cylinders is defective.
A further possible way of calculating an injected fuel quantity is to calculate the fuel quantity from detected impact times of the nozzle needle.
In addition, it is possible to calculate an injected fuel quantity by using the output signals of a cylinder pressure sensor.
Furthermore, WO 2010/003780 A1 discloses taking into account a change in the exhaust gas temperature as a function of the introduced fuel quantity in order to evaluate the function of an injection system.
EP 1 570 165 B1 discloses a method in which what is referred to as a minimum quantity is calculated by using selective actuation of small fuel quantities and observing the effect on the rotational speed signal.
This principle of action of actuating small quantities and evaluating the effects on the rotational speed signal can also be used if a brief ballistic actuation of an injector is replaced by actuation of a stable needle partial-stroke.
The two abovementioned methods assume that it is known from measurements at the test bench what fuel quantity leads to what change in the rotational speed signal. The primary objective is respectively to compensate changes in idle stroke or carbonization of an injector. However, as a result the quality of the fuel is also indirectly taken into account.
As already stated above, it is, however, possible for identical fault phenomena in the system to have different causes. A changed characteristic of the effect of small injection quantities can be caused, if there is no fault in the involved sensors, by incorrect injection quantities, incorrect injection times, changed fuel properties (ignition delay, energy content) and by faults in the fresh air system and exhaust gas recirculation system (influence of the combustion peak temperature). If a temperature sensor which is used for the assessment is not seated directly in the exhaust manifold of the engine but instead, for example, is only seated downstream of the oxidization catalytic converter a possibly changed catalytic efficiency level (exothermic) of the oxidization catalytic converter must then also be taken into account.
An incorrect injection quantity can in turn be caused by carbonization of the injection nozzles, further faults in the injector or else by an incorrectly measured fuel pressure. Customary fuel metering calculations convert a quantity setpoint value into an opening duration of the injection nozzle, specifically as a function of the measured fuel pressure and an assumed fuel density. In fact, a setpoint volume is therefore actuated, not a setpoint mass flow rate. A changed throughflow rate during the carbonization therefore likewise leads to a quantity error, as does an incorrectly measured fuel pressure.
In known systems, the cause/effect chains are varied. A problem with the identification of the cause of a fault which occurs is that there is no actual fixed point present. This applies, in particular, in the case of an on-board diagnosis. Basically, any component may be faulty. Previous approaches to a solution only achieve inadequate success in identifying the cause of faulty behavior of the internal combustion engine.